Miniature RF calibrator utilizing multiple power levels

ABSTRACT

A small light-weight battery operated calibrator device provides a precise sine wave output for use in calibration of test equipment, such as a RF Power Meter or a Spectrum Analyzer. The calibration device includes two power levels, one −40 dBm and one 0 dBm. The purpose of the two power levels is to obtain a slope and offset for correction of the RF power measuring device being calibrated. Operation indication LED lights are provided to indicate which of the two powers are in use, and if battery power is below acceptable levels. Miniature low power components including a crystal oscillator and a divide by 2 integrated circuit that generates a precise square wave and a low pass filter for converting the square wave into a precise sine wave allows the calibrator to be battery operated and stored as a calibration component.

CLAIM FOR PRIORITY

This application is a divisional of application Ser. No. 11/856,325filed on Sep. 17, 2007, now U.S. Pat. No. 7,683,602 entitled “MiniatureRF Calibrator Utilizing Multiple Power Levels,” by Donald AnthonyBradley, the entire contents of which are incorporated herein byreference.

BACKGROUND

1. Technical Field

The present invention relates to components used in the calibration orverification of absolute frequency and amplitude measuring testequipment. More particularly the present invention relates to a highlyaccurate sine wave generation circuit used in calibrating or verifyingthe accuracy of test equipment such as RF power meters and spectrumanalyzers.

2. Related Art

Existing calibration devices that can generate precise sine waves aretypically provided internal to the instrument being calibrated, or as anexternal attachment. The internal RF calibrator in a test devicetypically has one power level and is accessible using the front panelspace on the instrument. It may be undesirable, however, to use thefront panel space which is typically in short supply on portableinstrumentation. An external bench top calibrator, further, is typicallybulky and requires a wall plug in line voltage for operation. Therequired line voltage may not be available for a field test instrument.

FIG. 1 shows a block diagram of components of a typical calibrator. Thesystem includes a precise oscillator 2, typically operating at 50 MHz.The output of oscillator 2 is provided to a level modulator 4 thatprovides a stable voltage output from the oscillator 2 as controlled bya feedback signal. The output of the level modulator 4 passes through anamplifier 6, low pass filter 8, and attenuator 22 to a test port 24. Theattenuator 22 is shown as a variable attenuator, allowing a user to setthe desired attenuation level. The amplifier 6 increases the output ofoscillator 2, while low pass filter 8 removes unwanted harmonics. Thevariable attenuator 22 is typically included in an external bench topcalibrator that connects to a test device, allowing a user to selectdifferent output levels as desired during calibration. As an alternativeto the variable attenuator 22, a fixed attenuator can be used. A fixedattenuator is more typically included on a calibrator that is internalto a test device.

The feedback signal to the level modulator 4 is provided from anamplifier 16. The feedback signal comes to an input of amplifier 16 fromthe output of the low pass filter 8 through a detector diode 10 andresistor 14. A filter capacitor 12 removes an AC component of thefeedback signal. A capacitor 20 enables amplifier 16 to function as anintegrator. A second input to the amplifier 16 is provided from avoltage reference 18. The voltage reference 18 has a voltage value setto control the desired input level of attenuator 22.

It would be desirable to provide components for a calibration devicethat can provide a precise sine wave with two power levels that does notuse up front panel space on an instrument being calibrated, is notbulky, and does not require a line voltage attachment.

SUMMARY

According to embodiments of the present invention, a calibrator isprovided that can generate precise sine waves and not suffer thedrawbacks of prior art devices.

The calibrator is a battery operated and provides two very precise sinewave outputs for use in calibration of amplitude or frequency measuringtest equipment. With battery power, a line voltage is not requiredduring testing. The calibrator further uses small light weightcomponents, so it can be easily transported and used in a field testarea, and will not use front panel space of an instrument beingcalibrated.

The calibration device includes two power switches connecting thebattery to a voltage regulator, one with attenuation of −40 dBm and onewithout at 0 dBm to provide two calibrated RF power levels. The purposeof the two power levels is to obtain a slope and offset for correctionof the RF sensor in a test device being calibrated.

The switches selecting either −40 dB or 0 dB of attenuation drive thevoltage regulator that powers a crystal oscillator. The oscillator thendrives a divide by two flip flop that generates a highly symmetricalsquare wave that has its amplitude controlled by the precisiontemperature corrected DC voltage regulator and its frequency stabilizedby the quartz temperature corrected oscillator. The output of the divideby two frequency divider is then directed through a voltage divider to alow pass filter. With the −40 dB switch, the output of the divide by twofrequency divider is connected to the low pass filter through a 10Kresistor providing a 100:1 reduction. With the 0 dBm switch, the outputof the divide by two frequency divider is provided to the filter throughan AND gate that has a 10 Ohm resistance in series with a 90 Ohmresistor to form a total 100 Ohm resistor. An additional 100 Ohmresistor forms a two to one voltage divider with this first 100 Ohmcombination to provide a two to one voltage division with a 50 Ohmoutput impedance to the low pass filter. With either the −40 dB or 0 dBswitches, the voltage divider provides a precise square wave to the lowpass filter with a matched source impedance of 50 Ohms.

The low pass filter then removes all of the harmonics of the square waveto provide a precise sine wave output. The low pass filter output isprovided through an attenuator and blocking capacitor to an outputterminal of the calibrator. Diode protection devices are provided todivert static discharge or high power input surges applied to the outputconnector. The overall combination of components can be built from lightweight low power components that still provide the precise sine waveoutput. Miniature low power components allow the calibrator to bebattery operated and stored as a calibration component after use.

In some embodiments, operation indication LED lights are provided toindicate the operation state of the calibrator. A green LED is connectedwith circuitry to provide two intensities depending on whether the −40dBm or the 0 dBm attenuator is in use. A blinking red light is furtherconnected with circuitry to indicate if battery power is belowacceptable levels.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details of the present invention are explained with the help ofthe attached drawings in which:

FIG. 1 shows a block diagram of components of a conventional calibrator;

FIG. 2 shows a block diagram of a miniature RF calibrator according toembodiments of the present invention; and

FIG. 3 shows components providing LED lights connected to give a user avisual indication of the state of operation of the calibrator of FIG. 1.

DETAILED DESCRIPTION

FIG. 2 shows a block diagram of a miniature RF calibrator according toembodiments of the present invention. The RF calibrator includes abattery 102 connected to two switches 104 and 106. The switch 104provides an attenuation factor of 1 or 0 dBm, while the switch 6provides an attenuation factor of 100 or −40 dBm. The purpose of the twopower levels is to obtain a slope and offset for correction of the RFsensor in a test device being calibrated.

The output of the switches 104 and 106 are connected to a voltageregulator 8. The output of the voltage regulator 8 provides powerdriving a quartz oscillator 10, a divide by two flip flop 12, and a twoinput AND gate. The oscillator 10 provides a highly accurate frequencyat twice the output frequency to the divide by two flip flop 12. Thesquare wave output has its amplitude controlled by the precisiontemperature corrected DC voltage regulator 108 and its frequencycontrolled by the quartz temperature corrected oscillator 110. Anexemplary voltage regulator 108 that provides for such temperaturecorrection is the Analog Devices ADP3336. An exemplary quartztemperature corrected oscillator 110 is the Kyocera K30-3C0-100.0000.

The symmetry of the square wave is controlled by a divide by 2 frequencydivider 112. The frequency divider 112 can be constructed withcomplementary CMOS transistors. An exemplary frequency divider 112 isthe Fairchild NC7SZ74. A slight resistance change of the outputtransistors in the frequency divider 112 over temperature is compensatedfor by the temperature dependant voltage regulator 108 to yield aconstant output square wave voltage under a fixed resistive load. Outputsymmetry is inherent due to the frequency divider 112 changing states ononly the positive going edge of the quartz oscillator.

The output of the frequency divider 112 has a low 10 Ohm impedance. The10 Ohm frequency divider 112 matches an impedance of the AND gate 114which appears as a 10 Ohm resistor. The AND gate 114 can be constructedusing complementary CMOS transistors similar to the frequency divider112. The AND gate 114 will provide a 10 Ohm resistance for both the 0and 1 produced output. An exemplary circuit for the AND gate thatprovides a 10 Ohm resistance is the Fairchild NC7SZ02. Although an ANDgate 114 is shown and described, other logic providing a Boolean AND canbe used.

With switch 104 used the output of the AND gate 114 is enabled. Theoutput of the 10 Ohm AND gate 114 is then provided through a 90 Ohmresistor 116. The total resistance of the series AND gate 114 and the 90Ohm resistor is then 100 Ohms. This 100 Ohm total resistance isconnected to a node 117 to another 100 Ohm resistance 118 that connectsto ground. This forms a 50 Ohm output impedance voltage divider to drivethe low pass filter 122.

During use of switch 106, the output of AND gate 114 is disabled. Thedisabled AND gate 114 provides a 10 Ohm resistance to ground. Withswitch 104 disabled, the output of the frequency divider 112 is providedthrough a 10,000 Ohm resistor 120 to node 117 to connect to the low passfilter 122. The attenuation factor of the voltage divider formed by the10 Ohm AND gate 114 in series with the 90 Ohm resistor 116 and the10,000 Ohm resistor 120 presents a 100:1 reduction of the precisionsquare wave available to the low pass filter 122 compared with thesignal available when switch 104 is enabled.

With either switch 104 or 106 used, the precision square wave from node117 now enters the low pass filter 122. Filter 122 removes all harmonicsof the fundamental frequency. The filter 122 is designed to present a 50Ohm output impedance at the desired output frequency. It is alsodesigned to accept slight variations on its input impedance withoutaffecting its output impedance. This can be accomplished at a singlefrequency of interest. With switch 106 enabled, the filter 122 outputfrequency is now a pure sine wave with an amplitude of −36.5 dBm. Thefilter 122 is followed by a fixed 3.5 dB attenuator 124. The finaloutput at terminal 128 is, then, a −40.0 dBm pure sine wave. A sourcematch is tightly controlled to provide the greater than 40 dB returnloss and a SWR<1.02 by precision design of the attenuator 124 and lowpass filter 122. Although specific attenuation values for the switches104 and 106, resistance values of resistors 116, 117, 118 and 120, ANDgate 114, and attenuation of attenuator 124 are given, these exemplaryvalues may be changed depending on desired design requirements.

A DC blocking capacitor 126 follows the attenuator 124. The DC blockingcapacitor 126 is used to reference the output to 0 volts DC. Theblocking capacitor 126 is further used to block any unintended DC frombeing applied to the calibrator output. Back to back diodes 130 and 132at the input to filter 122 also prevent unintended RF energy as well asstatic discharge from destroying CMOS device components. The CMOScomponents that could be damaged include those in the AND gate 114 orthe frequency divider 112. A first diode 130 in the back to back diodesconnects node 117 at the input of filter 122 to ground, while the diode132 connects node 117 to the battery 102. Neither diode conducts currentduring normal operation.

Operation of the calibrator of FIG. 2 is described as follows.Depressing the −40 dBm push button enables power to the circuit anddisables AND gate 114. The quartz oscillator 110 produces a very stablefrequency at 2 times the output frequency. This signal has no amplitudecontrol or duty cycle control, but is suitable to drive the divide by 2divider 112. The output of divider 112 has a low 10 Ohm impedance. Theslight resistance change of the output transistors in the divider 112over temperature is compensated for by the temperature dependant voltageregulator 108 to yield a constant output square wave voltage into afixed resistive load. Output symmetry is inherent due to the frequencydivider 112 changing states on only the positive edge of the quartzoscillator 110.

With AND gate 114 disabled when using switch 106, the square wave isthen presented to the approximately 10,000 Ohm resistor 120 and thedisabled AND gate 114 and 90 Ohm resistor 116. Disabled AND gate 114appears as a 10 Ohm resistor to ground. The attenuation factor of thisvoltage divider represents a 100:1 reduction of the precision squarewave available at the output of divider 117 compared with the signalavailable when switch 104 is enabled. The precision square wave nowenters low pass filter 122 which filters all harmonics of thefundamental frequency. The filter 122 presents a 50 Ohm output impedanceat the desired output frequency. Filter 122 also accepts slightvariations on its input impedance without affecting its outputimpedance. This can be accomplished at a single frequency of interest.

With switch 106 enabled, the output of filter 122 is now a pure sinewave with an amplitude of −36.5 dBm. The filter 122 is followed by afixed 3.5 dB attenuator 124 and has DC blocked by capacitor 126. Thefinal output is a −40.0 dBm pure sine wave. The blocking capacitor 126references the output to 0 VDC. It also blocks any unintended DC frombeing applied to the calibrator output. Back to back diodes 130 and 132at the input to filter 122 prevent unintended RF energy as well asstatic discharge from destroying its CMOS components.

Depressing the 0 dBm switch 104 enables the AND gate 114. The output ofthe AND gate 114 is a precision square wave switching between ground andthe regulated voltage. It has a 10 Ohm output resistance, which inseries with the approximately 90 Ohm resistor 116 appears at 100 Ohms.The slight resistance change of the output transistors in the AND gate114 over temperature is compensated for by the temperature dependantvoltage regulator to yield a constant output square wave voltage into afixed resistive load. This 100 Ohms is provided in series with the 100Ohm resistor 118 to ground and creates a divide by two voltage dividerat node 117. The Thevinin equivalent impedance of the input of filter122 then appears as a fixed 50 Ohms for both 0 and −40 dBm selections,and further operation of the calibrator is similar to that describedwith the −40 dBm switch depressed.

FIG. 3 shows components providing dual colored LED lights 206 and 228connected to give a user a visual indication of the state of operationof the calibrator of FIG. 2. Depressing the −40 dBm switch button 106causes the green LED 206 to illuminate at a visible brightness.Depressing the 0 dBm switch button 104 causes the green LED 206 toilluminate twice as bright. Battery voltage below a usable range neededto keep the regulator 108 in regulation causes the red LED to flash 228,indicating a low battery condition for battery 102. In one embodiment,the LED lights 206 and 228 can be provided by a single red/green LED. Anexample of such a red/green LED is the Lumex SSL-LX30591 GW.

The state indication circuit includes a comparator amplifier 201 havinga first input connected to the output of voltage regulator 108, and asecond input connected through a voltage divider formed by resistors 220and 222 to the input of voltage regulator 108. Power is supplied to thecomparator 201 from the input to the voltage regulator. The output ofcomparator 201 drives a resistor 204 that connects to the green LED 206.An exemplary circuit for the comparator is the National SemiconductorsLMV7239. Under normal conditions the comparator 201 provides an outputof logic one or the voltage of battery 102. To increase the intensity ofthe green LED 206 when switch 104 is depressed, a PMOS FET transistor208 is provided with a gate connected to the ground connection of theswitch 104. An exemplary PMOS FET transistor 208 is the Zetex ZXM61P02F.With the switch 104 depressed, the source-drain path of transistor 208connects the output of comparator 201 through a resistor 210 to thegreen LED 206, thus reducing the overall resistance from the output ofcomparator 201 and LED 206 and increasing intensity of LED 206. Withswitch 104 open, the transistor 208 will remain off and the intensity ofLED 206 will be reduced when switch 106 is connected.

An oscillator 224 is connected by a resistor 226 to the red LED 228. Theinput of the oscillator 224 receives a disable signal from the output ofcomparator 201. Thus, when the oscillator 124 is not receiving a disablesignal from comparator 201, it will enable the oscillator 224 and thered LED 228 will blink on and off at the oscillator 224 frequency ofapproximately 10 Hertz. For convenience, components in FIG. 3 that arecarried over from FIG. 2 are similarly labeled.

Operation of the circuitry of FIG. 3 used in driving the green LED 206is described as follows. First, selection of the −40 dBm switch 106 andsufficient voltage from battery 102 for proper operation will illuminatethe green LED 206 at moderate brightness. The selection of 0 dBm switch104 and sufficient battery voltage enables the boost transistor 208 thatapplies approximately twice the current to the green LED 206 so that itprovides twice the illumination.

Operation of the circuitry used in driving the red LED 228 is describedas follows. First, the voltage regulator 108 provides a referencevoltage used to compare to the voltage of the battery 102. If thevoltage of battery 102 drops below approximately 0.2V above the voltageof regulator 108 output the comparator 201 will change state from a 1 toa 0. This will enable the 10 Hz flashing oscillator which drives the redLED 228. The green LED 206 will be disabled.

Although specific voltages for battery 102, oscillation frequencies forthe LEDs, and LED colors are described, these are exemplary and may bechanged based on design requirements.

Although the present invention has been described above withparticularity, this was merely to teach one of ordinary skill in the arthow to make and use the invention. Many additional modifications willfall within the scope of the invention, as that scope is defined by thefollowing claims.

1. A calibration device for providing a test signal to calibrate an RFPower Meter or a Spectrum Analyzer, the calibration device comprising: abattery power connection; a temperature compensating voltage regulatorhaving an input and an output; a first switch for connecting the batteryconnection to a voltage regulator; a second switch for connecting thebattery connector to the voltage regulator, the second switch providinga different attenuation than the first switch from the batteryconnection to the voltage regulator; an oscillator having a power supplyinput connected to the output of the voltage regulator and an output,the oscillator providing a first signal at its output; a frequencydivider having an input connected to the output of the oscillator andhaving an output, the frequency divider having a power supply inputconnected to the output of the voltage divider; a logic gate providingan AND operation, the logic gate having a first input connected to theoutput of the oscillator, a second input connected by the second switchto a ground connection, and an output, the logic gate having a powersupply input connected to the output of the voltage divider; a low passfilter for converting the first signal from the oscillator to the testsignal for providing to an output of the low pass filter; a firstresistor for connecting the output of the frequency divider to the lowpass filter; a second resistor for connecting the output of the logicgate to the low pass filter; a third resistor connecting the low passfilter to ground; an attenuator having an input connected to the outputof the low pass filter and providing an output; and a DC blockingcapacitor connecting the output of the attenuator to the output of thecalibration device.
 2. The calibration device of claim 1, furthercomprising a state indication device comprising: a first LED connectedto the output of the voltage regulator to provide a first lightingcondition when the first switch is connected to the battery supplyconnection and a second lighting condition when the second switch isconnected to the battery supply connection.
 3. The calibration device ofclaim 2, wherein the state indication device further comprises: ancomparator having a first input connected to the output of the voltageregulator, a second input, and an output, and having a power supplyinput connected to the input of the voltage regulator; a voltage dividerconnecting the input of the voltage regulator to the second input of thecomparator; a first LED resistor connecting the output of the comparatorto an input of the first LED; a second LED resistor having a firstterminal connected to the input of the first LED and having a secondterminal; and a transistor having a gate connected to the second inputof the logic gate, and a source-drain path connecting the output of thecomparator to the second input of the second LED resistor.
 4. Thecalibration device of claim 3, further comprising: a second LED having asecond light, the second LED flashing on and off when a voltage at thebattery supply connection drops below a desired level.
 5. Thecalibration device of claim 4, wherein the state indication devicefurther comprises: an oscillator having an enable signal input connectedto the output of the comparator, and an output; and a third LED resistorconnecting the output of the oscillator to the second LED.
 6. Thecalibration device of claim 3, further comprising: a second LEDproviding a lighting condition indicating when the battery supplyconnection drops below a desired level.
 7. The calibration device ofclaim 6, wherein the first LED and the second LED have different colors,wherein the different lighting conditions of the first LED comprisesdifferent intensities, and wherein the lighting condition of the secondLED indicating a low battery supply comprises flashing of the LED.